Typically, tandem color printers include four photosensitive drums for yellow, magenta, cyan, and black colors. These photosensitive drums, as a whole, can be attached to and detached from the main body casing of the color printer. This type of color printer includes a drawer frame capable of moving into and out from the main body casing. The four photosensitive drums are arranged on the drawer frame. The drawer frame is provided with four cartridge mounting portions corresponding to the respective photosensitive drums, on which toner cartridges are detachably disposed to be in one-to-one correspondence with the photosensitive drums. The toner cartridge includes a developing roller and contains toner therein. When the toner cartridge becomes empty with toner, the toner cartridge is removed from the drawer frame and replaced with a new one.